ci: update Cannon CI job with explicit medium resource class, and updated gotestsum parallel flag

.circleci/config.yml

@@ -62,6 +62,7 @@ jobs:
   cannon-go-lint-and-test:
     docker:
       - image: us-docker.pkg.dev/oplabs-tools-artifacts/images/ci-builder:latest
+    resource_class: medium
     steps:
       - checkout
       - check-changed:
@@ -82,7 +83,7 @@ jobs:
           name: Cannon Go tests
           command: |
             gotestsum --format=standard-verbose --junitfile=/tmp/test-results/cannon.xml \
-            -- -parallel=8 -coverpkg=github.com/ethereum-optimism/optimism/cannon/... -coverprofile=coverage.out ./...
+            -- -parallel=2 -coverpkg=github.com/ethereum-optimism/optimism/cannon/... -coverprofile=coverage.out ./...
           working_directory: cannon
       - run:
           name: upload Cannon coverage